Open Access Dissertation
Date of Award
Within the numerous fields of inorganic chemistry, there is a growing interest to design dinucleating ligands for transition metals. This interest derives from the expansive list of bimetallic and polymetallic centers observed in nature that employ vital small molecules for catalysis, such as carbon dioxide, dinitrogen, and water. Many research groups have attempted to harness that structural and functional metal-metal cooperativity displayed by these enzymes, in hopes of enhancing transition metal catalysis. The overall focus of my dissertation is on utilizing late first-row transition metals, bound to either iminopyridine, bis(imino)pyridine, or bis(2-pyridinylmethyl)amine chelates, tethered by a xanthene linker towards bimetallic cooperativity in catalysis and small molecule activation. The specific goals of my research were to: (1) investigate the effect of ligand flexibility and metal-metal distance in alkyne cyclotrimerization catalysis; (2) investigate the cooperative activation of small molecules by xanthene-based bimetallic systems. Three projects were proposed and undertaken to accomplish these goals.
The first project focuses on initial attempts to catalyze alkyne cyclotrimerization reactions, beginning with the treatment of a dinucleating bis(iminopyridine) ligand L1, bearing a xanthene linker, (L1 = N,N′-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) with Ni2(COD)2(DPA) (COD = cyclooctadiene, DPA = diphenylacetylene). This reaction leads to the formation of a new dinuclear complex Ni2(L1)(DPA). Ni2(L1)(DPA) can also be obtained in a one-pot reaction involving Ni(COD)2, DPA and L1. The X-ray structure of Ni2(L1)(DPA) reveals two square-planar Ni centers bridged by a DPA ligand. DFT calculations suggest that this species features NiI centers antiferromagnetically coupled to each other and their iminopyridine ligand radicals. Treatment of Ni2(L1)(DPA) with one equivalent of ethyl propiolate (HCCCO2Et) forms the Ni2(L1)(HCCCO2Et) complex. Addition of the second equivalent of ethyl propiolate leads to the observation of cyclotrimerized products by 1H NMR spectroscopy. Carrying out the reaction under catalytic conditions (1 mol% of Ni2(L)(DPA), 24 h, room temperature) transforms 89% of the substrate, forming primarily benzene products (triethyl benzene-1,2,4-tricarboxylate and triethyl benzene-1,3,5-tricarboxylate) in 68% yield, in a ca. 5 : 1 relative ratio. Increasing catalyst loading to 5 mol% leads to the full conversion of ethyl propiolate to benzene products; no cyclotetramerization products were observed. In contrast, the reaction is significantly more sluggish with methyl propargyl ether. Using 1 mol% of the catalyst, only 25% conversion of methyl propargyl ether was observed within 24 h at room temperature. Furthermore, methyl propargyl ether demonstrates the formation of cyclooctatetraenes in significant amounts at a low catalyst concentration, whereas a higher catalyst concentration (5 mol%) leads to benzene products exclusively. Density functional theory was used to provide insight into the reaction mechanism, including structures of putative dinuclear metallocyclopentadiene and metallocycloheptatriene intermediates.
The second project likewise targets alkyne cyclotrimerization reactions, forgoing nickel and iminopyridine ligands, while focusing on the application of dicobalt octacarbonyl (Co2(CO)8) as a metal precursor in the chemistry of formally low-valent cobalt with redox-active bis(imino)pyridine [NNN] ligands. It is known that Co2(CO)8 serves as a catalyst for various cyclotrimerization reactions. The treatment of both mononucleating mesityl-substituted bis(aldimino)pyridine (L3) and dinucleating macrocyclic xanthene-bridged di(bis(aldimino)pyridine) (L2) with Co2(CO)8 were investigated. Independent of the metal-to-ligand ratio (1 : 1 or 1 : 2 ligand to Co2(CO)8), the reaction of the dinucleating ligand L2 with Co2(CO)8 produces a tetranuclear complex [Co4(L2)(CO)10] featuring two discrete [Co2[NNN](CO)5] units. In contrast, a related mononucleating bis(aldimino)pyridine ligand, L3, produces different species at different ligand to Co2(CO)8 ratios, including dinuclear [Co2(CO)5(L3)] and zwitterionic [Co(L3)2][Co(CO)4]. Interestingly, [Co4(L2)(CO)10] features metal–metal bonds, and no bridging carbonyls, whereas [Co2(CO)5(L3)] contains cobalt centers bridged by one or two carbonyl ligands. In either case, treatment with excess acetonitrile leads to disproportionation to the zwitterionic [Co[NNN](NCMe)2][Co(CO)4] units. The electronic structures of the complexes described above were studied with density functional theory. All the obtained bis(imino)pyridine complexes serve as catalysts for cyclotrimerization of methyl propiolate, albeit their reactivity is inferior compared with Co2(CO)8.
The final project concerns the cooperative activation of carbon dioxide and other related heteroallenes, focusing on the synthesis of di-copper systems featuring bis(2-pyridinylmethyl)amine chelates bridged by a xanthene linker. The treatment of a dinucleating ligand L6 with tetrakis(acetonitrile)copper(I) hexafluorophosphate led to the formation of [Cu2L6][PF6]2 in CH2Cl2 and formation of the theoretical [Cu2(NCCH3)2L6][PF6]2 species in acetonitrile. [Cu2L6][PF6]2 displays near C2v symmetry in solution, with two methylene proton resonances observed by 1H NMR spectroscopy, and C1 symmetry in the solid-state, with pseudo-linear coordination geometry for one copper center and distorted tetrahedral geometry for the adjacent copper center. [Cu2(NCCH3)2L6][PF6]2 also exhibits near C2v symmetry in solution, with only one methylene proton resonance observed by 1H NMR spectroscopy. Neither di-copper system reacts with carbon disulfide or carbon dioxide in their respective solvents at room temperature over a 24-hour period. Future studies will focus on exploring alternative reaction conditions.
Hollingsworth, Ryan Lance, "Synthesis Of Open-Chain And Macrocyclic Xanthene-Bridged Bimetallic Cobalt, Nickel, And Copper Complexes For Cooperative Catalysis" (2018). Wayne State University Dissertations. 2106.